Reactions during conversion of simplified low-activity waste glass feeds

Abstract The mechanisms that affect the incorporation of Tc-99, a volatile radioactive component of concern, into glass melt during vitrification of low-activity waste (LAW) are being investigated to develop a method to increase the retention of Tc-99 in glass waste form. Previous studies with simulated LAW glass feeds (slurry mixture of liquid waste and chemical/mineral additives) demonstrated that the early stage feed-to-glass conversion reactions below 800 °C are critical for the retention of Re (used as a nonradioactive surrogate of Tc-99) in glass. To examine the effect of feed composition on the feed-to-glass conversion reactions, simplified systems containing major LAW components (NaNO3 and NaOH) and representative additive components (SiO2 and H3BO3) were designed and tested. The ratio of H3BO3 to NaNO3 was varied in a three-component system without NaOH and the ratio of NaOH to NaNO3 was varied in a four-component system at a fixed H3BO3 to NaNO3 ratio. As a first step of testing with simplified feeds, this study applied thermal analyses and phase characterization of the reacting feeds, which were performed without the addition of Re, to investigate the evolution of salt phases during the slurry drying process and upon heating of dried feeds.